Protecting Thermocouples and RTD Sensors from Damage

Thermocouples function on the principle of the Seebeck effect, which states that two dissimilar metals combined at two junctions produce an electromotive force, or EMF, at the junctions. The metals react to variations in temperature to create an EMF voltage in relation to the difference in temperature at the junction. RTDs function on the precept that electrical resistance increases with rising temperature. The types of metal used to fabricate the sensor impact accuracy, response time, measurement range and resistance to environmental stressors such as vibration.

Grounded Junction, OMEGACLAD^® Probes

Ungrounded Junction, OMEGACLAD^® Probes

Thermocouple junctions may be ungrounded or grounded. They are frequently covered with protective metal but may be left exposed to enhance response time. Grounding is often required to prevent accumulation of static charge, which may negatively impact accuracy. However, if the thermocouple is grounded to machinery or other electrically powered equipment, circuit noise may hinder the measurement. Many different metal combinations are used in the construction of thermocouples. Each is classified based on temperature range and suitable measuring environments. Thermocouples enclosed in metal are quite robust and, on average, a lot less vulnerable to vibration than RTDs.

1PT100G Glass Wire-Wound RTDs

RTDs are available in thin-film or wire-wound types. Wirewound sensors are extremely accurate. They are made by winding copper, platinum or nickel wire around a ceramic or glass core to which the wire is also fused. Glass-core sensors can be immersed in a majority of liquids without protection while those with a ceramic core provide stability for remarkably high temperature measurements. Platinum is the most favored wire, since it offers the best accuracy over the widest temperature range.

ASTM E1137 is the global standard that defines tolerances for platinum resistance sensors. It is often used as one of the criteria for selecting a temperature sensor, as RTDs manufactured and tested according to this specification offer superior reliability and improved performance.

TFD Thin-film RTD shown magnified

Thin-film RTDs offer considerably more vibration protection than wire-wound RTDs. They are created by placing a thin film of passivated platinum on a ceramic substrate. An electrical circuit is etched into the material to generate the preferred resistance. These sensors display a practically linear temperature-resistance curve. Thus, they provide very accurate and consistent measurements over a wide temperature range. Their compact size gives them the benefit of quicker response times and greater resistance to thermal shock and vibration.

Challenges Presented to Temperature Measurements in the Presence of Vibration

Vibration can create mechanical stress in the wires of RTDs and thermocouples. Thermocouples are subject to vibration fatigue, which can result in short circuits and insulation failure. This may be apparent from intermittently high readings resulting from the measurement being taken at the short rather than at the junction. Wire-wound RTDs are particularly vulnerable to vibration damage. The fine platinum wire used to wind the sensor has a characteristic diameter of 15 to 35 µm and is quite delicate. A damaged or broken RTD sensor wire may result in:

• Noisy signals
• An open circuit
• Sporadically high temperature measurements

Decalibration is another fault condition that may take place in thermocouples exposed to vibration. This is the process whereby the structure of the wire is changed to where the voltage-temperature features no longer comply with international standards. The main concern with decalibration is that the temperature measurements seem to be accurate. The readings will drift slowly over time. Testing the thermocouple against a known temperature is the most common technique of detecting decalibration.

Types of Vibrations that Affect Sensors

Machine vibrations are typical in industrial processes. They can arise from the movement of pumps, motors or compressors. The tendency to cause damage is proportional to the frequency and amplitude of the vibration. The amplitude is the force being applied to an object that is creating the vibration. For example, the rotational speed in an electric motor will add to the amplitude of vibration. The faster the motor rotates, the bigger the amplitude. Frequency is also an element in the severity of vibration. It is the rate at which a mechanical device travels back and forth under force. A machine can vibrate in many directions with differing rates of amplitude and frequency.

Acoustical vibrations are produced by a large number of mechanical systems, such as engines and turbines, as well as vehicle traffic and human voices. When acoustical noise enters a structure, it becomes structural vibration. Sound waves can move anywhere there is air flow; thus, they can arise from any direction. Reverberation is the continuance of sound after the original has stopped. This is the outcome of sound waves reflecting from surfaces. Acoustic features can differ according to the shape and size of objects they reflect from, making it hard to predict how they will react.

Flow-induced vibrations result from the interaction of forces between fluid flow and the inertia of structures immersed in or carrying it. Fluid flow is a source of energy capable of creating mechanical and structural vibration. In cylindrical structures, vibrations are classified as either cross-flow-induced or axial-flow induced, based on the angle of inward flow in relation to the cylinder axis.

Vibration Resistant Thermocouples and RTDs

The OMEGA PR-21SL RTD is built for use in thermowells and features spring loading to withstand contact between the probe and the thermowell in spite of static and vibration. This guarantees optimal heat transfer between the probe and thermowell and insulates the sensor against vibration. The PR-21SL RTD can be used in two-, three- or four-wire applications and fits standard 0.26-inch bore thermowells. A modifiable, self-gripping spring allows it to be used in shorter thermowells.

OMEGA’s PR-31 RTD probe is vibration resistant and bendable. The probe is built of 316 stainless steel and the mineral-insulated cable allows the probe to be bent. The PR-31 RTD is vibration tested to MIL-STD-202G, Method 204D, Condition A and has a measuring range between -50 and 500 °C. It is offered in 100 and 1000 Ω and can be used in 2-, 3- or 4-wire applications.

The M12M Series thermocouple probes can be used exposed, mounted into the process, or in a thermowell. The probes are offered as a Type K thermocouple with Inconel 600 sheaths or a Type J with 304 stainless steel sheaths. The Type K has a temperature range between -40 and 1150 °C, and the Type J has a temperature range of -40 to 600 °C. The M12M is delivered as standard with an ungrounded junction; a grounded junction is optional.

PR-21SL RTD

PR-31 RTD Probe

M12M Thermocouple Probe

Conclusion

Selecting the correct RTD or thermocouple for an application will enhance performance and prevent sensor damage. Thermocouples are a multipurpose and cost-effective means of temperature measurement and provide the best protection against vibration. Wire-wound RTDs offer better accuracy and a wider measurement range but are not as robust. Thin film RTDs provide very accurate and consistent data and provide greater resistance to vibration than wire-wound RTDs. OMEGA also has tailor-made engineered solutions for extremely severe vibration environments.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.